Rheology modifiers are used generally to adjust or modify the rheological properties of aqueous compositions. Such properties include, without limitation, viscosity, flow rate, stability to viscosity change over time and the ability to suspend particles in such aqueous compositions. The particular type of modifier used will depend on the particular aqueous composition to be modified and on the particular end-use of that modified aqueous composition. Examples of conventional rheology modifiers include thickeners such as cellulosic derivatives, polyvinyl alcohol, sodium polyacrylate and other watersoluble macromolecules, and copolymeric emulsions in which monomers with acid groups have been introduced onto the main chain.
Another class of rheology modifiers known to thicken aqueous compositions is one typically referred to as associative modifiers. Such associative modifiers are reported in U.S. Pat. Nos. 4,743,698, 4,600,761, RE 33,156, 4,792,343, 4,384,096, 3,657,175, 5,102,936 and 5,294,692. As noted, these "alkali-swellable" thickeners become effective upon the addition of base, thereby raising the pH of the thickened composition to alkaline, but the thickeners do not thicken aqueous compositions having acidic pH. These types of thickeners also are believed to be incompatible in systems containing cationic ingredients.
Other rheology modifiers which are "activated" by the addition of acid to aqueous compositions which contain the modifiers also have been reported. As reported, emulsions are prepared via free-radical emulsion polymerization utilizing colloidal stabilizers. The emulsions are mixed with the composition to be thickened and then acid is added to the mix, thereby lowering the pH of the system to 6.5 to 0.5. These thickeners are reported to be effective at thickening certain acidic aqueous compositions, but are not effective at thickening aqueous compositions having basic pH.
It would be desirable to develop a polymeric rheology modifier which is stable to change in viscosity and phase separation over time when in the form of an emulsion, and which advantageously may be used to thicken both acidic and basic compositions.